1. Field of the Invention
The invention relates to a nuclear magnetic flowmeter for measuring the flow rate of a medium flowing through a measuring tube, with a magnetization apparatus for magnetization of the medium flowing through the measuring tube over a magnetization distance along the longitudinal axis of the measuring tube, the magnetization apparatus for producing the magnetic field which is used to magnetize the medium being provided with permanent magnets and having at least two magnetization segments which are located in succession in the direction of the longitudinal axis of the measuring tube.
2. Description of Related Art
The atomic nuclei of the elements which have a nuclear spin also have a magnetic moment which is caused by the nuclear spin. The nuclear spin can be construed as an angular momentum which can be described by a vector, and accordingly, the magnetic moment can also be described by a vector which is parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus in the presence of a macroscopic magnetic field is aligned parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of the precession is called the Larmor frequency ωL and is proportional to the amount of the magnetic field strength B. The Larmor frequency is computed according to ωL=γ·B. In the latter γ is the gyromagnetic ratio which is maximum for hydrogen nuclei.
Measurement and analysis methods which use the property of the precession of atomic nuclei with a magnetic moment in the presence of a macroscopic magnetic field are called nuclear magnetic resonance measurement or analysis methods. Usually, the voltages induced into a sensor coil by the processing atomic nuclei under various boundary conditions are used as the output variable for the measurement and analysis methods. One example for measuring instruments which use nuclear magnetic resonance is the nuclear magnetic flowmeters which measure the flow rate of a multiphase medium flowing through the measuring tube and which analyze the medium.
The prerequisite for an analysis using nuclear magnetic resonance is that the phases of the medium which are to be analyzed can be excited to distinguishable nuclear magnetic resonances. The analysis can comprise the flow velocities of the individual phases of the multiphase medium and the relative proportions of the individual phases in the multiphase medium. Nuclear magnetic flowmeters can be used, for example, to analyze the multiphase medium extracted from oil sources. The medium consists essentially of the crude oil, natural gas and salt water phases, all of which contain hydrogen nuclei.
The medium extracted from oil sources can also be analyzed with so-called test separators. Test separators branch off a small part of the extracted medium, separate the individual phases of the medium from one another and determine the proportions of the individual phases in the medium. However, test separators are not able to reliably measure proportions of crude oil smaller than 5%. Since the proportion of crude oil of each source continuously drops and the proportion of crude oil of a host of sources is already less than 5%, it is not currently possible to economically exploit these sources using test separators. In order to also be able to exploit sources with a very small proportion of crude oil, correspondingly accurate flowmeters are necessary.
It is immediately apparent from the equation for computing the Larmor frequency ωL that the Larmor frequency ωL is proportional to the amount of magnetic field strength B of the macroscopic magnetic field in the medium to be studied and thus the amount of the magnetic field strength also acts directly on the frequency of the voltage which has been induced into the sensor coil. The direction of the macroscopic magnetic field with reference to the orientation of the sensor coil also influences the voltages induced in the sensor coil. In general, deviations of the macroscopic magnetic field which is permeating the medium from the ideal of the homogeneous magnetic field lead to a reduced measurement quality and thus to inaccurate measurement results.
Desired and known gradients of the magnetic field in the medium are expressly accepted from these unwanted deviations.
Examination of magnetic fields with gradients is omitted since the following statements can obviously be applied to magnetic fields with gradients.
U.S. Pat. No. 7,872,474 B2 discloses a nuclear magnetic flowmeter on which the invention is based. It applies to the magnetization elements which belong to the magnetization apparatus that they are made hollow-cylindrical and have a homogeneous magnetic field in their interiors. The magnetization segments are arranged in succession on the measuring tube such that their concentric longitudinal axes coincide with the longitudinal axis of the measuring tube. The magnetization of the medium flowing through the measuring tube can be set differently, therefore can be varied by the homogeneous magnetic fields of the individual magnetization segments being aligned either parallel or anti-parallel to one another.
FIGS. 7(a)-7(c) of U.S. Pat. No. 7,872,474 B2, in particular, show a magnetization apparatus with six successively arranged magnetization segments. In the implementation according to 7(a) all magnetization segments are set such that the homogeneous magnetic fields of the individual magnetization segments are aligned parallel to one another in the medium. Conversely in the implementation according to 7(b), three magnetization segments at a time are combined into a group. Within each group the homogeneous magnetic fields of the magnetization segments are aligned parallel to one another. But, the homogeneous magnetic fields of one group are aligned anti-parallel to the homogeneous magnetic fields of the other group. Finally, according to 7(c), again, two groups of magnetization segments are also formed, but one group with four magnetization segments and the other group with two magnetization segments. It also applies here that the homogeneous magnetic fields of the individual magnetization segments in each group are aligned parallel to one another, the homogeneous magnetic fields of the individual magnetization elements of one group, however, being aligned anti-parallel to the homogeneous magnetic fields of the magnetization segments of the other group.